1. Field of the Invention
The present invention relates to an exposure apparatus and an image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus forms an electrostatic latent image by exposing a photosensitive member to light emitted from a light source and develops the electrostatic latent image by toner to thereby form an image. One known method of exposing a photosensitive member involves causing light from a light source to scan the photosensitive member by a polygonal mirror and forming an electrostatic image on the rotating photosensitive member by the scanning light. A further known method involves arraying a plurality of light sources such as LEDs (light-emitting diodes) along the axis of rotation of a photosensitive member and causing these light sources to emit light to thereby form an electrostatic image on the photosensitive member.
In order to improve resolution and raise the speed of image formation, a driving circuit for driving a light-emitting element at a high driving frequency is required. In general, an emitter follower is employed in the driving circuit of the light source of the exposure apparatus. In accordance with the specification of Japanese Patent Laid-Open No. 8-72293, an arrangement in which two transistors are switched alternatingly is illustrated. A control signal for controlling an exposure light source is input to one of the transistors. Connected to the other transistor is a resistor the resistance of which is approximately equal to that of the light source, and a signal the polarity of which is opposite that of the control signal flows into this other transistor. The object of such a system is to utilize only the transistor turn-on characteristic. The driving circuit is arranged in such a manner that a current from a current source is conducted to the resistor in a case where the light source is not made to emit light. Furthermore, according to Japanese Patent Laid-Open No. 8-72293, an independent connection circuit is provided between a pulse modulating circuit and the exposure light-source driving circuit in order to prevent slowing of the control signal.
The exposure apparatus used in an ordinary printer is equipped with a laser and a polygonal mirror. In order to attain a higher speed, however, an available method dispenses with the polygonal mirror and uses a number of light-emitting elements [LEDs or VCSELs (Vertical Cavity Surface Emitting Lasers)]. With this method, it is possible to scan and expose a plurality of lines simultaneously by driving a number of light-emitting elements simultaneously.
In order to drive a number of light-emitting elements in this manner, however, it is necessary to take into consideration the arrangement of the light-emitting elements and the driving circuit elements that drive the light-emitting elements. An apparatus according to the prior art is shown in FIGS. 6A and 6B. As illustrated in FIG. 6A, light-emitting elements 202a, 202b, 202c, 202d and driving circuit elements 201a, 201b are mounted on a board 200. FIG. 6B is a diagram that focuses on the light-emitting element 202a and driving circuit element 201a extracted from FIG. 6A. As illustrated in FIG. 6B, a resistor R, a first current path 303 and a second current path 304 are mounted on the board 200. The driving circuit element 201a has transistors Tr1, Tr2 and an inverter 302. The transistors Tr1, Tr2 form a common-collector circuit, and the collectors of the two transistors are connected to a power source. A signal from an input terminal 301 is input to the base of the transistor Tr1, and a signal obtained by inverting the signal from the input terminal 301 is input to the base of the transistor Tr2. A signal based upon input image data is input to the input terminal 301.
In a case where the light-emitting element 202a is lit (e.g., in a case where an H-level signal is input from the input terminal 301), current from the current source flows into the first current path 303. On the other hand, in a case where the light-emitting element 202a is not lit (e.g., in a case where an L-level signal is input from the input terminal 301), current from the current source flows into the second current path 304.
Thus, since the conventional apparatus has a small number of light-emitting elements (exposure light sources), the light-emitting elements 202a, 202b, 202c, 202d and the driving circuit elements 201a, 201b can be mounted on the same board 200. As a result, as shown in FIG. 6B, the second current path 304 that conducts current from the current source to the resistor R in a case where the light-emitting element 202a is not lit can be placed in the neighborhood of and adjacent to the first current path 303 that conducts the current from the current source to the light-emitting element 202a in this case. In a case where the light-emitting element is turned ON and OFF (lit and extinguished) at a high frequency in such an arrangement, the frequencies of the voltages impressed upon the first and second current paths are of opposite polarity. As a consequence, radiant noise produced by the first current path takes on a polarity opposite the radiant noise of the second current path so that the radiant noise is cancelled out.
In a case where the number of light-emitting elements is increased in order to raise the speed of image formation, the board on which the light-emitting elements and the driving circuit elements are mounted must be enlarged. However, the space available for installing a board in a scanning apparatus is limited and there are instances where the light-emitting elements and driving circuit elements cannot be mounted on the same board.
In such cases the light-emitting elements (202a to 202n) and driving circuit elements (201a to 201n) are mounted on different boards, as illustrated in FIG. 7. However, when the light-emitting element 202a and driving circuit element 201a are mounted on separate boards, the first current path 303 and second current path 304 in FIG. 6B become more widely spaced apart and it is difficult for the radiant noise produced by both current paths to be cancelled out. In particular, different light-emitting elements (202b to 202n) and their driving circuit elements (201b to 201n) exist in the vicinity of the light-emitting element 202a and driving circuit element 201a, respectively, and there is the danger that such radiant noise will be picked up. When radiant noise is picked up, the light-emission state of the light-emitting elements becomes destabilized and stable exposure of the photosensitive member can no longer be carried out.